This invention relates to techniques for generating focus and tracking error signals in an optical storage system.
Optical disk systems use a focused laser beam to read or write data on a storage medium. These systems may incorporate read-only, write-once, or erasable storage technology. A common component of such systems is an objective lens disposed over the storage medium (typically a disk) which collects radiation from a light source (typically a laser) and focuses it onto the selected data track in the medium. The character of the light reflected from the medium is indicative of the data recorded in the selected data track. In a typical system, the reflected light is routed to a sensor which, in combination with detection circuitry, recovers this data.
Information indicative of the precision of the focus and tracking of the light source can also be obtained from the light reflected from the medium. To use this information, a typical optical disk system also routes the reflected light to a focus and tracking sensor, which relays error signals to an active control system which servos the focus and tracking of the laser beam.
Several methods for independently sensing focus and tracking errors have been employed. Among these are the biprism and astigmatic methods. The biprism method is the more sensitive, but the astigmatic method offers a useful tradeoff between focus error sensitivity and acquisition range. However, both methods require the manufacture of optical components of unusual shape.